Axinite-series

Chemical formula: Ca₂(Fe,Mg,Mn)Al₂BSi₄O₁₅[OH]

A group of silicates characterized by a vitreous luster and occurrence as flattened, sharp-edged crystals with distinctive colors.

## Characteristics The axinite series includes four distinct minerals (axinite-(Fe), axinite-(Mg), axinite-(Mn), tinzenite) that form solid solutions, meaning their chemical composition changes continuously. All are characterized by a similar, very distinctive crystal habit – they are flattened, tabular, with sharp edges, resembling an axe blade in shape. Crystal surfaces often show striations. Specimens are usually transparent or translucent, with a strong, vitreous luster. ## Physical Properties Axinites are characterized by significant hardness, ranging from 6.5 to 7 on the Mohs scale, allowing them to scratch glass. Their density ranges from 3.26-3.36 g/cm³, depending on the dominant element in their composition (iron, magnesium, or manganese). The luster is strongly vitreous, and these minerals may exhibit weak pleochroism, which is a change in color depending on the direction of observation. ## Colors and Varieties The color of minerals from the axinite series is an important feature and depends on their chemical composition: - **Axinite-(Fe)** (ferroaxinite) is the most common and has a characteristic clove-brown, violet-brown, or brownish-black color. - **Axinite-(Mn)** (manganaxinite) takes on shades from yellowish-orange to brown, less commonly reddish. - **Axinite-(Mg)** (magnesioaxinite) is the rarest, usually pale, bluish, gray, or light brown. - **Tinzenite** is an intermediate form between axinite-(Mn) and axinite-(Fe) and has a color from yellow to orange-brown. ## History and Name The name "axinite" comes from the Greek word "axine" (ἀξίνη) meaning axe, which is a direct reference to the typical, axe-like shape of the crystals. The mineral was first described by the French mineralogist René Just Haüy in 1797. ## Uses Minerals from the axinite series are primarily valued as collector's items. Exceptionally clear and well-colored crystals, especially axinite-(Fe) and axinite-(Mn), are sometimes faceted and used as rare gemstones. They have no significant industrial application.

Properties

Mohs hardness: 6.5-7
Luster: Vitreous
Streak: White
Density: 3.26-3.36
Cleavage: Good on {100}, distinct on {001}, {110} and {011}
Fracture: Conchoidal to uneven
Transparency: Transparent to translucent
Crystal system: Triclinic

Diagnostic features

## Identification The most important diagnostic feature of axinite is its unique, flattened, axe-shaped crystal habit with sharp edges. Also characteristic is its clove-brown color (for axinite-(Fe)), strong vitreous luster, and high hardness (6.5-7). Crystals often exhibit striations on their faces. This mineral also shows pyro- and piezoelectric properties, meaning it generates an electric charge under temperature change or pressure. ## Distinguishing from Similar Minerals Axinite is sometimes confused with some other minerals: - **Titanite (Sphene)**: May have similar colors but forms crystals with a different, wedge-shaped habit and has a higher luster, often approaching adamantine. - **Epidote**: Often occurs in the same environments but usually forms prismatic, elongated crystals with a characteristic pistache-green color. - **Smoky Quartz**: May have a similar color but crystallizes in hexagonal prisms terminated by pyramids and has a slightly lower density. ## Crystal Forms Axinites crystallize in the triclinic system, forming characteristic, flattened (tabular) crystals with sharp edges. They often occur as rosettes, fan-shaped aggregates, or radial aggregates. They can also form granular and massive aggregates.

Geological environment

## Genesis Minerals from the axinite series are formed mainly through contact and regional metamorphism. They occur in skarns, which are rocks formed at the contact of a magmatic intrusion (most often granitic) with carbonate rocks (limestones, dolomites). They also occur in fissures and Alpine-type veins within greenschists, amphibolites, and gneisses, as well as in some pegmatites. ## Mineral Associations Axinite often co-occurs with minerals such as: quartz, calcite, epidote, clinozoisite, vesuvianite, diopside, hedenbergite, garnets (mainly andradite and grossular), titanite, prehnite, zeolites, and sulfides, e.g., pyrite and chalcopyrite. ## Localities The most important localities for axinites worldwide include: - **France**: The vicinity of Bourg d'Oisans in the Alps, a historical type locality for axinite-(Fe), providing classic, brown crystals. - **Russia**: Polar Urals (Puyva), known for large and well-formed axinite-(Fe) crystals. - **USA**: New Melones Lake mine in California, famous for excellent, violet-brown axinite-(Fe) crystals. Also deposits in Nevada and New Jersey (Franklin), where axinite-(Mn) occurs. - **Mexico**: San Martín mine in Zacatecas and the Santa Rosa area in Baja California. - **Pakistan and Afghanistan**: Mountainous regions supply gem-quality specimens. - **Tanzania**: Merelani Hills, known for the occurrence of axinite-(Mg). - **Switzerland**: Numerous localities in the Alps, where tinzenite and other axinites occur in Alpine fissures.

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp-edged crystals of high transparency and intense, attractive color. Large, single crystals or aesthetic crystal groups set on a contrasting rock matrix are particularly sought after. Specimens from classic, historical localities, such as Bourg d'Oisans, are highly rated. For faceted stones, clarity (lack of inclusions and fractures), color intensity, and cut quality are important. ## Market Prices Axinite prices vary widely. Small, single crystals or fragments can be purchased for a few tens of Polish zlotys. Good quality specimens with distinct crystals on matrix cost from several hundred to several thousand Polish zlotys. Exceptional, large, and aesthetic crystal groups from renowned localities can reach prices of several tens of thousands of Polish zlotys or more. ## Popular Localities The most desired axinite-(Fe) specimens by collectors come from Puyva in the Urals (Russia), the New Melones Lake mine (California, USA), and the classic locality of Bourg d'Oisans (France). For axinite-(Mn), specimens from Franklin (New Jersey, USA) are highly valued. Gem-quality specimens often come from Pakistan and Tanzania.

Care and storage

## Cleaning Axinite specimens are best cleaned with lukewarm water and a mild soap, using a soft brush. Ultrasonic and steam cleaners should be avoided, as they can damage crystals, especially if they contain internal fractures. After washing, they should be thoroughly rinsed, preferably in distilled water, and allowed to dry completely. ## What to Avoid Axinite is sensitive to rapid temperature changes (thermal shock), which can cause fracturing. Contact with strong acids, especially hydrofluoric acid, which can etch the crystal surface, should be avoided. Prolonged exposure to intense sunlight may, over time, lead to fading of some specimens, particularly those with more delicate colors. ## Storage Due to its relatively high hardness, axinite is quite resistant to scratching, but its sharp edges can be prone to chipping. It is recommended to store specimens individually, in separate boxes or on stands, to avoid contact with other minerals. They should be protected from dust and direct sunlight.

Sources

IMA Mineral List (CNMNC)